1. Field of the Invention
The invention is related to magnetic storage devices, and in particular, to determining the quality of an erase process for perpendicular magnetic recording (PMR) disks.
2. Statement of the Problem
Of the many magnetic storage devices, the hard disk drive is the most extensively used to store data. Hard disk drives may include a number of different types of hard disk recording media. One type of hard disk recording media used in hard disk drives is longitudinal magnetic recording. In longitudinal recording, the axis of magnetization is oriented parallel with the underlying hard disk substrate. In another type of recording media, called perpendicular magnetic recording, the axis of magnetization is oriented perpendicular to the underlying hard disk substrate. During the manufacturing of the hard disk drive, hard disks are fabricated and then bulk erased using a variety of processes, such as insertion of the hard disks into a magnetic tunnel. The erase process is designed to pre-condition and prepare the disk for subsequent magnetic recording. Different types of pre-conditioning are desired depending on the recording media used. In a PMR disk, an alternating magnetic field state (AC erased state) is desired. A high quality erase process of the PMR disk before assembly is an important consideration in the manufacturing process, as it directly affects the final manufacturing yield of the hard disk drive. Portions of the disk which contain non-alternating magnetic field states (DC erased states) can saturate the read head in the hard disk drive, which results in distorted servo signals from the read heads and reduced hard disk drive yield.
One previous method of determining the quality of the erase process of the PMR disk before assembly involves optical testing of the surface of the PMR disk. In optical testing, a PMR disk is visually inspected under magnification to detect specific visual patterns. Based on the patterns, the quality of the erase process is inferred. One problem with optical testing is that it is not very reliable in determining the erase state of the PMR disk. After the erase process, the PMR disk is assembled into the hard disk drives. Hard disk drives which fail a post-assembly erase quality test require a lengthy in-drive erase process, which is time consuming and expensive to the manufacturing process.